https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79173
--- Comment #13 from Andrew Pinski <pinskia at gcc dot gnu.org> --- (In reply to Vincent Lefèvre from comment #12) > One issue is that _addcarry_u64 / x86intrin.h are not documented, so the > conditions of its use in portable code are not clear. I suppose that it is > designed to be used in a target-independent compiler builtin. None of the x86 intrinsics are documented except on the Intel intrinsics guide page: e.g: https://software.intel.com/sites/landingpage/IntrinsicsGuide/#text=_addcarry_u64